Mixed culture of tilapia (Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) in periphyton-based ponds

In recent years, the concept of periphyton-based aquaculture has been tested and applied in aquaculture. Positive effects of substrate addition for periphyton development included increasing the food supply and providing shelter for culture animals. The aim of this project was to develop a low-cost culture technology for resource-poor farmers in South-East Asia. The technology is an extension of traditional brush-park fisheries in which bacteria, protozoa, fungi, phytoplankton, zooplankton, benthic organisms and a range of other invertebrates colonized the substrates installed in rivers and lakes, attracting many fishes. These microbial communities are an excellent food for fishes or crustaceans. The effects of the addition of substrates to ponds to stimulate periphyton development have been studied intensively, but for tilapia-prawn polyculture the methodology still had to be optimized. This thesis focused on the effects of substrate and artificial feed addition on the overall nutrient dynamics, growth, production and economics of tilapia-prawn polyculture. A stepwise approach was followed.</span>The first step was to quantify the effects of substrate and prawn addition to tilapia ponds on total productivity and to quantify the contribution of substrates to tilapia and prawn production. In the second step, the best tilapia<span class=GramE>:prawn</span>stocking ratio for periphyton ponds was determined</span><span lang=EN-US style='mso-ansi-language:EN-US'>. The third step was to optimize the stocking density in periphyton ponds while using the previously determined best stocking ratio. In the fourth step, the optimized</span><span lang=EN-GB style='mso-ansi-language:EN-GB'>tilapia-prawn polyculture package was compared to a traditional (fertilizer, no feed, no substrate), a fed traditional and a periphyton-based fed production systems in terms of total fish production and economic benefits.<span style='mso-spacerun:yes'> </span>The final step combined all data from the previous experiments into a multivariate analysis looking at</span><span lang=EN-US style='mso-ansi-language:EN-US'>the relationships between production and pond ecology related parameters.<o:p></o:p></span>In the first study, a considerable overlap was found between the feeding niches of tilapias and prawns (Chapter 2). Substrate addition improved the food conversion ratio in tilapia ponds by 32%, while stocking of prawns resulted in an additional 12% improvement. On average, substrate addition resulted in a 40% higher net yield of tilapia in monoculture and 56% in tilapia-prawn polyculture. The individual weight gain of tilapia increased by 30% due to addition of substrates in both mono- and polyculture ponds. Substrate addition resulted in 9% higher survival and 45% higher tilapia production while prawn survival increased 75% and production 127%. The highest total yield (2445 kg ha ^-1 tilapia and 141 kg ha ^-1 prawn) over a 145 days culture period was recorded in periphyton-based tilapia-prawn polyculture ponds. The positive effects on survival and production of tilapia of additional prawn stocking indicated mixed culture of these two species is promising and options for further optimization should be explored.<o:p></o:p></span>In Chapter 3, the tilapia<span class=GramE>:prawn</span>stocking ratio in the periphyton-based production system was optimized. The survival of tilapia was higher in polyculture whereas that of prawn was higher in monoculture. The periphyton biomass decreased with increased stocking density of tilapia, indicating the preferential feeding of tilapia on periphyton.<span style='layout-grid-mode:line'>The highest production (1,623 kg tilapia and 30 kg prawn ha ^-1 ) was recorded in the combination of 75% tilapia and 25% prawn at a total density of 20,000 ha ^-1 .</span>The cost benefit analysis revealed that the monoculture of tilapia and addition of prawn to the tilapia ponds at any ratio were profitable.<o:p></o:p></span>Chapter 4 explored the optimum stocking density of tilapia and prawn in periphyton-based polyculture ponds. Total stocking densities of 20,000, 30,000 and 40,000 individual ha ^-1 were tested. Periphyton biomass on the substrates increased during the first 2 months of culture and then decreased for all stocking densities. Survivals of tilapia and freshwater prawn were higher at low and medium (83-86% for tilapia and 51-57% for prawn) stocking densities than at high (78% for tilapia and 39% for prawn) stocking density. The combined net yield of tilapia and freshwater prawn were higher at medium (2,372 kg ha ^-1 ) and high (2,303 kg ha ^-1 ) than at low (1,641 kg ha ^-1 ) stocking density. The net profit margin was highest (69%) at medium (30,000 ha ^-1 ) and lowest (44%) at high (40,000 ha ^-1 ) stocking density.<o:p></o:p></span>Chapter 5, compared tilapia-prawn production in non-fed periphyton-based ponds with traditional (fertilizer, no feed, no substrate), fed traditional and periphyton-based fed ponds. The food conversion ratio (FCR) of tilapia was 13% lower in periphyton-based fed ponds than in fed traditional ponds. Both substrate and feed addition influenced tilapia harvesting weight but not prawn harvesting weight. Survival, final weight gain and net yield of both tilapia and prawn were highest in periphyton-based fed ponds and lowest in traditional ones. The absence of significant differences in survival, harvesting weight and net yield of both tilapia and prawn between periphyton-based and feed driven ponds indicate that periphyton is a good alternative to supplemental feeding. The combined net yield was 59% higher in periphyton-based fed ponds, 48% in fed traditional and 47% in non-fed periphyton-based ponds compared to traditional ponds.<span style='mso-spacerun:yes'> </span>The net profit margin was similar between fed traditional and non-fed periphyton-based ponds.</span><span lang=EN-US style='mso-ansi-language:EN-US'>Interestingly, although the contribution of prawn to the total production in quantity, as compared to tilapia, was very low (4-7%), it contributed 22-32% to the total sales value. Therefore, even at low density, freshwater prawns contributed an important fraction of farming revenue in tilapia-prawn periphyton-based ponds.</span><span lang=EN-GB style='mso-ansi-language:EN-GB'><o:p></o:p></span>In Chapter 6, all water quality, periphyton and fish production data from the previous four experiments were merged into a single dataset and re-analysed using multivariate statistics, to gain deeper insights into the functioning of the ecosystem. The objective was to explore the effects of tilapia and prawn density, presence of substrates for periphyton growth and supplemental feed on the water quality in tilapia-prawn polyculture ponds. The main sources of water quality variability were due to photosynthesis, nutrient uptake, respiration and decomposition. Substrates for periphyton development resulted in a more favorable environment for the cultured organisms and provided an extra source of food for culture animals. The positive effects on the overall pond ecology supported the conclusions from the previous studies that tilapia<span class=GramE>:prawn</span>polyculture in periphyton-based ponds is a reliable production system that improves farming benefits considerably.<o:p></o:p></span>In the final discussion (Chapter 7), the applicability of this new technology was reviewed, giving attention to developmental aspects and contemplating ideas for future research